We usually think of transcription factors as controlling the level of gene expression, but we can also view the expression level as being the cell's measurement of the TF concentration [1]}, [2]}. As outside observers of the cell, we can measure the concentration of transcription factors with considerable accuracy [3]}.
However the cell's “measurement” of TF concentration is based on the arrival of these molecules at their binding sites, and this is a noisy process, because TF concentrations are low, in the nanoMolar range [4]}, [5]}, [6]}, [7]}, [8]}.
Physical limits to the measurement of such low concentrations were first explored in the context of bacterial chemotaxis [9]}, but have proven to be much more general [1]}, [11]}, [12]}, [13]}. What will be important for our discussion is not the precise values of these limits, but rather that the limits exist and are significant on the scale of biological function.
